Control system for modulating water heater

ABSTRACT

A control system is provided for a modulated heating system including a plurality of modulating water heaters, which may be modulating boilers. A deadband control scheme provides for reduced cycling of the modulating heater when total system heat demand falls between the maximum output of one heater and the sum of the maximum output of that one point and the minimum firing point of the next subsequent heater.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to water heaters and boilers,and more particularly, but not by way of limitation, to a control systemfor a modulating water heater or boiler which is particularlyconstructed for use as one of a plurality of water heaters or boilerscontrolled in a cascading sequence.

2. Description of the Prior Art

Conventional water heater technology utilizes a burner designed tooperate at a fixed flow rate of combustion air and fuel gas to theburner. Such a water heater cycles on and off in response to a controlsystem which monitors the temperature of water in a storage tank orelsewhere in various conduits of the water supply system. One example ofsuch a typical prior art system which is presently being marketed by theassignee of the present invention is that shown in U.S. Pat. Nos.4,723,513 and 4,793,800 to Vallett et al., the details of which areincorporated herein by reference.

It has been recognized that, in circumstances where there is asubstantially varying demand for heat input to the water supply system,greater energy efficiencies can be achieved through the use of a waterheater which is capable of operating at different energy inputs. Oneexample of such a system is that sold by Lochinvar Corporation, theassignee of the present invention, under the trademark COPPER-FIN II®.The Lochinvar COPPER-FIN II® system utilizes a plurality of stagedburners which can be brought on-line or taken off-line as the demand forheat energy changes. The COPPER-FIN II® unit includes multiple banks,for example, first, second, third and fourth stages. It initially turnson all four stages of burners, and as it approaches the desiredtemperature, it sequentially shuts off units to decrease the inputenergy. This type of system provides variable input, but it is notcontinuously variable. Instead the input can be changed only insubstantial increments corresponding to the heat output of one burnerstage.

The prior art has also included proposals for water heaters havingcontinuously variable output over a range of outputs. Two such systemsare shown in U.S. Pat. No. 4,852,524 to Cohen and U.S. Pat. No.5,881,681 to Stuart. These systems, which have been marketed by AercoInternational, Inc. under the BENCHMARK name, utilize a nozzle mixburner which receives independent streams of combustion air and fuelgas. A fuel/air valve is utilized to simultaneously control the flow ofair through the air line and fuel through the fuel line so as to providea varying input of fuel and air while maintaining a constant fuel to airratio. The blower speed remains constant on these systems.

More recently the assignee of the present invention has developed acontinuously variable water heater with variable air and fuel input, asshown in U.S. Pat. No. 6,694,926 to Baese et al. In the Baese apparatusa variable flow blower provides premixed combustion air and fuel to theburner at a controlled blower flow rate within a blower flow rate range.This allows the heat output of the water heater to be continuouslyvaried within a substantial flow range having a turndown ratio of asmuch as 4:1.

Various systems for controlling a plurality of modulating boilers aredescribed in U.S. Pat. No. 5,042,431 to Shprecher et al., includingsystems in which the last boiler turned on is the modulating boiler.

In large commercial operations it is common to utilize a plurality ofcommonly controlled heat exchangers such as those of the Vallett et al.patents or the Baese et al. patent described above. A number of uniqueproblems are encountered when using such heaters in groups, and thepresent invention is directed to improved control systems for suchboilers.

SUMMARY OF THE INVENTION

The present invention provides a control system particularly suited foruse with a plurality of modulating water heaters, which may be boilers,arranged for control in a cascade sequence wherein a first boiler isbrought online at its firing point and is then continuously modulated upto its maximum output, and then the first boiler is maintained at itsconstant output while firing a second boiler which is then modulatedfrom its firing point up to its maximum output as the overall heatdemand on the system increases. In similar manner, each boiler isbrought up to its maximum output before the next adjacent boiler isfired, and all previously fired boilers are maintained at maximum outputwith the modulation for the system coming from modulation of the lastfired boiler.

One challenge encountered with a system of modulating boilers like thatjust described is to minimize the cycling on and off of each boiler. Aswill be appreciated by those skilled in the art, the various componentsof the boiler and control system encounter more wear in cycling on andoff than in constant operation, and particularly items such as the hotsurface igniter are susceptible to requiring maintenance if they arecycled an excessive number of times. The present invention reduces thiscycling by addressing several issues encountered in certain operatingconditions with such a system.

In one aspect of the invention a unique control routine is provided forthose situations wherein the overall heat demand for the system of aplurality of boilers falls within a zone lying between the maximum heatoutput of the first boiler and the sum of the maximum heat output of thefirst boiler and the minimum heat output of the next adjacent boiler.This zone, which may be referred to as a deadband or dead zone, presentsunique operational problems because the second boiler cannot modulatewithin that range. This problem is addressed by allowing the watersupply temperature which is being sensed by the control system to varywithin a defined temperature range spanning a temperature set pointbefore the second boiler is turned on or off. Once the second boiler isturned on it is maintained at a constant output, preferably its minimumoutput, until the water supply temperature reaches or exceeds the upperend of the defined temperature range. The defined temperature range ispreferably the temperature set point plus or minus a constant. Theconstant lies in the range of from 3° F. to 7° F. and is preferablyabout 5° F.

Another issue encountered in commercial heating systems is that the flowrate of the water in the primary heating loop can change frequently andby a significant amount as heating zones are turned on and off. As thisflow changes, the heat load on the system typically changes with it. Inprior art systems the only indication the boiler has that heat load haschanged is when one of the temperature sensors detects a change insystem temperature. But the change in system temperature lagssignificantly in time behind the change in flow rate, and causesundesirable swings in system temperature and on and off cycling of theboiler as the boiler control system attempts to correct for the changein temperature. This problem is addressed by monitoring the flow ratewithin the system and then utilizing an algorithm to predict the changein heat load which will result from that change in flow rate, andvarying the firing rate of the boiler to correspond to the expectedchange in heat load. This allows the boiler to react much more quicklyto changes in system demand and significantly reduces temperature swingsin the system and reduces cycling of the boiler itself.

A third issue encountered in these multiple boiler systems is theproblem of condensation of water vapor in exhaust flue gases. Suchcondensation is acidic, and therefore if condensation is expected muchmore expensive Category IV venting materials are required by codes, withseparate flue vents for each boiler. On the other hand, if thetemperature of the exhaust flue gases can be reliably controlled andmaintained above a temperature at which the water vapor will condensefrom the exhaust gases, then much more economical Category I materialsmay be utilized and a common exhaust flue may be used for multipleboilers. This is particularly a problem with modulating boilers andsystems of modulating boilers because as the power demands change andthe power output of each boiler changes its exhaust gas temperaturechanges. This problem is addressed by providing an exhaust gastemperature sensor in each boiler and controlling the modulation rangeof each boiler during modulation so that the boiler is not allowed tooperate below a minimum heat output, which is typically significantlygreater than the minimum firing point for the boiler, and is high enoughto maintain boiler exhaust gas temperatures at the required level. Thiswill reduce the practical modulation range for each boiler, thusincreasing the width of the deadbands noted above.

Accordingly it is an object of the present invention to provide improvedcontrol systems for modulating water heaters.

Another object of the present invention is the provision of an improvedcontrol system for a modulating water heater utilizing premixed air andgas.

Still another object of the present invention is the provision ofcontrol systems for pluralities of modulating water heaters arranged ina cascading control sequence.

Still another object of the present invention is the provision ofcontrol systems which will reduce cycling of modulating water heaters.

Yet another object of the present invention is the provision of acontrol system that reduces cycling of a modulating water heater when aplurality of heaters are called upon to provide a combined heat outputwhich falls in a deadband between the modulating range of one heater andthat of the next subsequent heater.

Still another object of the present invention is the provision of acontrol system which allows early detection of changes in system heatload by sensing changes in water flow before those changes have had timeto significantly impact water temperature.

Yet another object of the present invention is the provision of controlsystems for preventing condensation of water vapor from exhaust fluegases in multiple water heater systems.

Other and further objects features and advantages of the presentinvention will be readily apparent to those skilled in the art upon areading of the following disclosure when taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of a single boiler or water heater ofthe present invention.

FIG. 2 is a schematic illustration of the individual control systemassociated with a single one of the boilers like that of FIG. 1.

FIG. 3 is a schematic elevation section view of the boiler of FIG. 1.

FIG. 4 is a schematic plan view of the boiler of FIG. 1 including theprimary hydraulic connections, the exhaust gas flue outlet and variousinternal sensors.

FIG. 5 is a schematic illustration of a plurality of boilers, in thisexample three boilers, operating together to provide hot water to avariable flow heating system.

FIG. 6 is a schematic illustration of the system heat demand as placedupon the three boiler system of FIG. 5, graphically illustrating thedeadbands in modulation range between each boiler and the nextsubsequent boiler in a cascading control sequence.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the drawings, and particularly to FIG. 1, the waterheater or boiler apparatus of the present invention is shown andgenerally designated by the numeral 10. As used herein, the term waterheater refers to an apparatus for heating water, including both steamboilers and water heaters that do not actually “boil” the water. Much ofthis discussion refers to the apparatus 10 as a boiler 10, but it willbe understood that this description is equally applicable to waterheaters that do not boil the water. The boiler 10 includes a heatexchanger 12 having a water side 14 having a water inlet 16 and a wateroutlet 18.

The general construction of the heat exchanger 12 may be similar to thatdisclosed for example in U.S. Pat. No. 4,793,800 to Vallett et al., orthat in U.S. Pat. No. 6,694,926 to Baese et al., the details of whichare incorporated herein by reference. As illustrated in FIGS. 2, 8 and 9of Vallett et al., the heat exchanger is a multiple pass exchangerhaving a plurality of fin tubes arranged in a circular pattern with aburner located concentrically within the circular pattern of fin tubes.This structure is schematically illustrated in FIG. 3 of the presentapplication wherein the heat exchanger 12 is shown to have upper andlower headers 20 and 22 connected by a plurality of vertically orientedfin tubes 24.

A burner 26 is concentrically received within the circular array of fintubes 24. The burner 26 is operatively associated with the heatexchanger 12 for heating water which is contained in the water side 14of the heat exchanger 12. As schematically illustrated in FIG. 4, thewater flowing through the water side 14 of heat exchanger 12 flows infour passes through the interior of the various fin tubes 24 between theupper and lower headers 20 and 22. Within each fin tube 24, the waterreceives heat from the burner 26 that is radiating directly upon theexterior fins of the fin tubes 24.

The burner 26 may be constructed in any suitable manner including thatdisclosed in Baese et al. U.S. Pat. No. 6,694,926, or in U.S. Pat. No.6,619,951 to Bodnar et al., or U.S. Pat. No. 6,428,312 to Smelcer etal., all of which are incorporated herein by reference.

The burner 26 is of the type referred to as a premix burner which burnsa previously mixed mixture of combustion air and fuel gas. In the systemshown in FIG. 1, a venturi 28 is provided for mixing combustion air andfuel gas. An air supply duct 30 provides combustion air to the venturi28. A gas supply line 32 provides fuel gas to the venturi 28. Theventuri 28, may for example be a model VMU680 provided by Honeywell. Agas control valve 33 is disposed in supply line 32 for regulating theamount of gas entering the venturi 28. The gas control valve 33 includesan integral shut off valve.

In order to provide the variable output operation of the burner 26 avariable flow blower 34 delivers the premixed combustion air and fuelgas to the burner 26 at a controlled blower flow rate within a blowerflow rate range. The blower 34 is driven by a variable frequency drivemotor 36 (see FIG. 1).

The gas line 32 will be connected to a conventional fuel gas supply (notshown) such as a municipal gas line, with appropriate pressureregulators and the like being utilized to control the pressure of thegas supply to the venturi 28.

The gas control valve is preferably a ratio gas valve for providing fuelgas to the venturi 28 at a variable gas rate which is proportional tothe flow rate entering the venturi 28, in order to maintain apredetermined air to fuel ratio over the flow rate range in which theblower 34 operates.

An ignition module 40 controls an electric igniter 42 associated withthe burner 26.

Combustion gases from the burner 26 exit the boiler 10 through acombustion gas outlet 44 which is connected to an exhaust gas flue 46.

As is further described below with reference to FIG. 5, the water inletand outlet 16 and 18 will be connected to a secondary flow loop 110 ofthe heating system, and as schematically illustrated in FIG. 4 there ispreferably a bypass 48 provided between the inlet and outlet 16 and 18with a three way control valve 50 provided in the bypass 48 for allowingsome recirculation of heated water when necessary to maintain an inlettemperature at water inlet 16 at a sufficiently high level to preventcondensation of water vapor from combustion gas products within theboiler 10.

A plurality of temperature sensors are located throughout the boilerapparatus 10 including sensor T₁ at the water inlet 16, sensor T₂ at thewater outlet 18, and sensor T₃ at the exhaust gas outlet 44.

A high temperature limit switch 52 is provided for shutting down theboiler 10 in the event water temperature within the boiler 10 exceeds apredetermined limit which could damage the boiler.

An air pressure switch 53 is connected to venturi 28 via lines 51 and 49to monitor flow through venturi 28.

The Controller

A controller 54 is provided for the boiler 10. FIG. 2 provides aschematic illustration of the main inputs to the controller 54.

The controller 54 senses temperature from the water inlet and outlettemperature sensors T₁ and T₂, and senses temperature of the exhaust gasoutlet sensor or flue sensor T₃.

A setpoint selection system 56 is provided in association with thecontroller 54 whereby an operator may input to the controller 52 thedesired water temperature which is desired to be seen in a heatingsystem to which the boiler will be connected, as is further describedbelow with regard to FIG. 5. The setpoint selector 56 may be directlyassociated with the boiler 10, or the setpoint may be fed to thecontroller 54 of the boiler 10 from a building thermostatic control orthe like.

The controller 54 will control a number of aspects of the boiler 10 inorder to achieve the desired heat output. One function of the controller54 is to control the flow rate of the blower 34. As previously noted,the blower 34 includes a fan which is driven by electric motor 36. Theelectric motor 36 is controlled by a variable frequency drive 58, whichis in turn controlled by the controller 54. As will be understood bythose skilled in the art, the variable frequency drive 58 varies thespeed of the electric motor 36 and thus the output of the blower 34 byvarying the frequency of an electrical power signal provided to theelectric motor 36.

Preferably the controller 54 and associated control components areselected so as to provide a blower turndown ratio of at least 2:1, andmore preferably at least 4:1. For example, with a blower turndown ratioof 2:1, the blower 34 would be operated within a range of from 50% ofthe maximum output to 100% of the maximum output. This provides acorresponding burner range of 50% to 100% of maximum burner output. Fora turndown ratio of 4:1, the blower 34 would be operated in a range offrom 25% of maximum to 100% of maximum output. Again, this variation inoutput is controlled by varying the frequency of the electrical signalsent by the variable frequency drive 58 to the electric motor 36 whichin turn drives the blower 34.

The blower flow rate is continuously variable within the defined flowrate range. It will be understood that the term “continuously variable”is used in contrast to a staged burner system like that for example ofthe Lochinvar COPPER-FIN II® system described above. These continuouslyvariable systems may in fact be variable in very small incrementsrelated to the digital nature of the control system, but for allpractical purposes, the flow rate is continuously variable between itsupper and lower limits.

A second function of the controller 54 is to control the position ofrecirculation valve 50 as previously described.

The controller 54 also operates in conjunction with the ignition module40 which controls the electric igniter 42 associated with burner 26.

As seen in FIG. 4, the boiler apparatus 10 includes an external housing60 in which the heat exchanger 12 is received. The controller 54 isprovided in the form of an integrated control board located within thehousing 60, and an operator interfaces with the controller 54 through anoperator interface board 62.

A Multiple Boiler Variable Flow Heating System

Referring now to FIG. 5, a variable flow heating system is thereschematically shown and generally designated by the numeral 100. Theheating system 100 will include a plurality of the boilers 10, and inthe embodiment illustrated in FIG. 5 there are three boilers which aredesignated as 10A, 10B and 10C.

The heating system 100 includes a primary flow loop 102 through whichheated water is circulated by one or more primary system flow pumps 104.The primary flow loop 102 is broken into a plurality of parallel zonessuch as 102A, 102B, 102C, etc., each of which includes a heating loaddesignated as 106A, 106B, 106C, etc., and each of which includes a zonevalve 108A, 108B, 108C, etc. for controlling flow into the respectivezone.

The primary flow loop 102, may for example be providing water to aheating system for heating various areas of a building, and the variousheating loads 106A, 106B, 106C, etc. may represent the radiatorscontained in each area of the building. Heat to a given area of thebuilding may be turned on or off by controlling the zone valves 108.Thus, as the radiator is turned on and off or as the desired heat isregulated in various zones of the building, the water flow permitted tothat zone by zone valve 108 will vary, thus providing a varying waterflow through the primary flow loop 102 and a varying heat load on thesystem 100.

The hot water is provided to primary flow loop 102 from secondary loop110 in which the boilers 10A, 10B and 10C are located. The secondaryflow loop 110 takes water from the primary flow loop 102 at watertakeoff point 112 and returns heated water to the primary flow loop 102at water supply point 114. A short joining portion 115 of primary flowloop 102 provides open communication between the water takeoff point 112and water supply point 114, and in normal operation the flow will bedefined by the flow through the primary flow loop 102 and much of thewater in primary flow loop 102 will flow directly from the pump 104 pastwater takeoff point 112 and water supply point 114 returning to thevarious zones of the primary flow loop 102 without going through theboilers. A portion of the water circulating in primary flow loop 102,however, will be drawn off to the boilers 10 so as to add further heatto the system.

The boilers 10A, 10B and 10C are connected in parallel within thesecondary flow loop 110, and each has a boiler feed pump 116A, 116B and116C which draws water from the water takeoff point 112 as needed anddirects it to its associated boiler.

The water outlets 18A, 18B and 18C of each of the boilers 10A, 10B and10C, respectively, are each connected to the secondary flow loop 110.

The system 100 is a closed system in that the water in primary loop 102is continuously recirculated and no significant amount of make-up wateris typically required.

The system is designed such that each of the subzones 102A through 102Fof the primary flow loop 102 have substantially equal heating loads 106and are designed to operate at a substantially constant pressure dropacross the heating load. A pressure transducer 118 measures the pressuredrop across the heating loads 106 and a signal is sent via line 120 topressure controller 122 which in turn sends an analog speed signal vialine 124 to a variable frequency drive 126 associated with the variablespeed system pump 104. Line 124 also communicates the speed signal tothe boiler 10A where it is input to the boiler controller 54 of FIG. 2.

A system supply temperature sensor 128 is located within the primaryloop 102 upstream of the heating loads 106 and a signal indicative ofthat system supply water temperature is communicated via line 130 to thecontroller 54 of boiler 10A.

The various flues 46A, 46B and 46C from the boilers are shown connectedas a common flue 46.

Although the system 100 shown in FIG. 5 is a closed loop system designedto use the hot water for heating purposes, it will be appreciated bythose skilled in the art that the same control system principlesdescribed herein are equally applicable to a system of water heaters,which may also be referred to as boilers, which heat water supplied to ahot water reservoir or reservoirs from which that water is consumed.Conventional makeup water facilities would be utilized to provide makeupwater to replace the water consumed from the reservoir.

System Control Based Upon Early Sensing of Changes in Water Flow in thePrimary Loop

In a commercial heating system having a general arrangement like that ofFIG. 5, the flow rate of water in the primary loop 102 can changefrequently and by a significant amount as the various heating zones 106turn on and off. As this flow changes, the heat load of the systemtypically changes with it. In a typical prior art system the modulationof the boilers is controlled in response to a sensed system supplytemperature sensor temperature such as would be sensed by a sensor suchas the sensor 128 shown in FIG. 5. In such a prior art system the onlyindication the boilers have that the heat load has changed is when thetemperature sensor detects a change in system temperature. If the heatload had dropped significantly, this could cause the sensed temperatureto rise significantly as well and the modulating boiler would be forcedto turn off. If the flow in the primary loop drops enough, it could evenfall below the flow rate through the boilers, causing recirculation ofhot water in the secondary flow loop 110 from point 114 to point 112 ina reverse direction from the normal flow through the connecting segment115. When this happens, the high temperature limit switches 52 can trip.In either case, the boiler would eventually need to restart. The moretimes a boiler is forced to cycle on and off, the more wear and tearoccurs on it and its components, and the shorter its life.

To overcome this problem, the control system of the present inventionreceives the signal from pressure controller 122 over line 124 which isindicative of the speed of the system pump 104 and thus is indicative ofthe flow through the primary loop 102. Thus changes in the speed signalindicate changes in the flow rate through the primary loop 102.Alternatively, a flow meter can be placed in the primary flow loop 102such as indicated at locations 132 or 134. In either case a signal suchas from flow meter 134 would travel over communication line 136 back tothe controller 54 of boiler 10A. With the use of either the pump speedsignal from pressure control 122 or the flow rate signal from flow meter132 or 134, the controller 54 can be described as sensing a parametercorresponding to a change in system flow rate, such sensed parameterbeing a parameter other than a change in system temperature. In responseto the sensed parameter, the controller 54 modulates the output ofboiler 10A or of whichever of the other boilers 10B or 10C is in amodulating mode.

As will be appreciated by those skilled in the art, when one knows thevarious hydraulic parameters of the heating system 100, an expectedchange in heat load resulting from turning one of the heating zones onor off can be calculated based upon the sensed change in flow throughthe primary flow loop 102.

In the example illustrated, the signal from pressure control 122 to thevariable frequency drive 126 is in the form of an electrical currenthaving a magnitude between 4 and 20 mA. For one particular system, thealgorithm correlating the change in flow rate to the expected change inheat demand from the system is in the form:

LF = (Ip − 4  mA)/16  mA${P(t)} = {{LF}*\{ {{K\; 1*( {{Ts} - {Ta}} )} + {K\; 2*( {\sum\limits_{t = 0}^{t = n}( {{Ts} - {Ta}} )} \}}} }$Where:

-   Ip=Pump speed signal (4-20 mA)-   LF=load factor-   t=time interval-   n=number of time intervals since start of heat demand-   P(t)=target speed of blower-   K1=proportional gain-   K2=integral gain-   Ts=setpoint temperature-   Ta=actual temperature.

By sensing the change in flow rate, which will occur well before thatchange in flow rate can fully impact system temperature, the boilers 10can be modulated in anticipation of changes in system temperature thatwould occur as a result of the change in system flow rate, therebyreducing changes in system temperature resulting from the change insystem flow rate and reducing on and off recycling of the boilers.

Improved Methods for Cascading Sequence of Boilers

In the system 100, one, two or all three of the boilers 10 may beoperating depending upon the heat demand from the system. And the systemcould have more than three boilers. The boilers are controlled in acascade arrangement such that a first boiler such as boiler 10A comes oninitially at its minimum firing rate, and is then modulated continuouslyup to its maximum firing rate. Then as system demand continues to climb,the second boiler 10B is turned on at its minimum firing rate and iscontinuously modulated until it reaches its maximum firing rate. If theheat demand is such that the entire combined output of both boilers oneand two is not required, the first boiler 10A will remain firing at itsmaximum firing rate and the second boiler 10B will be modulated.Similarly if the system heat demand exceeds the capacity of the twoboilers 10A and 10B, the third boiler 10C will be fired and it willmodulate.

Referring now to FIG. 6 a graphical illustration is thereshown of theoperation of the three boilers as heat demand increases. The graph ofFIG. 6 represents heat demand on its vertical scale.

As will be appreciated by those skilled in the art, a gas fired boilerhas a minimum firing point greater than zero. Typically the firing pointof a gas fired boiler might be for example 25% of its maximum output.Thus a single boiler when fired provides at least 25% of its maximumoutput, and can be modulated between 25% and 100% of its maximum output.

Thus, as shown in FIG. 6 the minimum heat output that can be provided isthe boiler number 1 minimum firing point. Boiler number one has amodulating range between its firing point and its maximum output. Thenthere is a first dead zone or deadband 138 between the maximum output ofboiler number 1 and the sum of the maximum output of boiler number 1 andthe firing point of boiler 10B. Then boiler 10B can modulate through itsmodulation range, and then there is a second dead zone 140 between thesum of the maximum output of boilers one and two plus the firing pointof boiler three. Then boiler 10C can modulate to its maximum outputwhich represents the maximum heat output of the system.

The problem areas for control of such a system fall in the two deadzones 138 and 140. In those zones it is not possible for the modulatingboiler to closely track the heat demand of the system, because themodulating boiler is either on at its minimum firing point or it is off.

With a typical prior art system wherein the controller is trying toclosely track the heat demand of the system by maintaining system watertemperature such as at temperature sensor 132 at some setpoint which hasbeen input to the controller, the system will cycle with an undesirablefrequency if the system heat demand happens to fall within one of thedead zones 138 and 140.

The present invention significantly improves control within the deadzones 138 or 140 by allowing the water supply temperature sensed at 132to vary at least within a defined temperature range 142, graphicallyillustrated in FIG. 6 as spanning the temperature setpoint 144, beforethe second boiler or whichever boiler is modulating is turned on or off.

The horizontal scale on FIG. 6 represents the control error which is thesetpoint temperature minus the actual sensed temperature, as indicatedin the two crosshatched areas within the dead zones 138 and 140 locatedbetween the upper and lower limits +T_(DB) and −T_(DB). Thus thecrosshatched areas represent the control error permitted in the deadzones. When the heat demand of the system is in one of the dead zones,and the control error is small, that is it falls between −T_(DB) and+T_(DB), the controller stops trying to adjust the output of the systemand the modulating boiler remains either on at its minimum firing pointor off. Eventually, when the control error exceeds the upper or lowerlimit ±T_(DB), the controller will resume adjusting the total output byturning the last modulating boiler either on or off.

Thus the operation of the system can be described as follows. When aheat demand upon the plurality of boilers falls within the modulationrange of the first boiler 10A, boiler 10A will be continuously modulatedto maintain its output such that the water supply temperature sensed at132 remains substantially equal to the temperature setpoint.

When the heat demand upon the plurality of boilers falls within thefirst deadband 138, the water supply temperature sensed at 132 isallowed to vary at least ±T_(DB) about the setpoint before the secondboiler is turned on or off. Thus, if only the first boiler is on, andthe system heat demand rises into the first dead zone 138, boiler 10Awill remain firing at its maximum load, and boiler 10B will not turn onuntil the system temperature drops below setpoint −T_(DB). After thatpoint boiler 10B will turn on at its minimum firing point and willremain firing at its minimum firing point until system temperatureexceeds the setpoint +T_(DB) after which point boiler 10B will againturn off. So long as total system demand remains in the first dead zone138, boiler 10B will turn on after sensed temperature at 132 drops tosetpoint −T_(DB), and boiler 10B will fire at its minimum firing pointuntil the sensed temperature at 132 exceeds setpoint +T_(DB).

More preferably, the boiler 10B is not turned on until systemtemperature falls below the setpoint −T_(DB) sufficiently that thesystem heat demand rises to the sum of the maximum output of boiler 10Aplus the minimum output of boiler 10B. Then the boiler 10B remains on atits minimum firing point until the sensed temperature at 132 exceedssetpoint +T_(DB) sufficiently that the system heat demand drops to themaximum output of boiler 10A. Also, the control system preferably“freezes” its calculation of heat demand during such time as the systemtemperature is within the range of setpoint ±T_(DB). After the systemtemperature goes outside that range, the controller again beginscalculating system heat demand.

If the system heat demand continues to rise to where the total heatdemand rises above the first dead zone 138 into boiler 10B's modulatingrange, then the control system will once again begin continuouslymodulating the output of boiler 10B to keep the temperature sensed at132 substantially equal to the setpoint.

If system demand then rises into the second dead zone 140, boilers 10Aand 10B will continue to fire at their maximum output, and boiler 10Cwill be turned on and off at its minimum firing point as the sensedtemperature drops below or rises above setpoint ±T_(DB). When totalsystem demand rises above the second dead zone 140 into the modulatingrange of boiler 10C, the control system will once again begin tocontinuously modulate boiler 10C to keep the temperature sensed at 132substantially at the setpoint.

As seen in FIG. 6, the temperature setpoint is preferably in the middleof the defined range 142. The defined temperature range is preferablyequal to the temperature setpoint plus or minus a constant. The constantwill typically be in the range of from about 3° F. to about 7° F., andpreferably the temperature constant is about 5° F. so that the definedrange 142 is preferably equal to the setpoint plus or minus 5° F.

The width of the dead zone temperature range is typically set in thesoftware of the controller 54 by the manufacturer or installer, and isnot typically adjustable by the user of the system.

It will be appreciated that each of the boilers 10 includes a controller54 and any one of the boilers 10A, 10B or 10C may serve as a lead boilerand its controller as the master controller. Preferably the role of leadboiler is periodically rotated between each of the boilers in the systemso as to substantially equalize the number of operating hoursexperienced by each boiler. Thus although the description abovegenerally refers to boiler 10A as boiler number 1 and describes thecontroller 54 of boiler 10A as performing the control function, it willbe understood that the role of lead boiler can rotate to any of theother boilers.

In general the controller 54 of whichever boiler is serving as leadboiler is operable to serve as a master controller to designate anoperating sequence of the boilers. The controller 54 includes a controllogic operable to define a combined operating range of the plurality ofboilers including a deadband such as 138 between a maximum output of oneboiler such as boiler 10A and the sum of the maximum output of the oneboiler 10A plus a minimum output of the sequentially next boiler 10B,the control system being operable when a system heat demand falls withinthe deadband to allow the sensed water temperature to vary within thetemperature range 142 spanning the temperature setpoint 144 before thesequentially next boiler 10B is turned on or off. The controllers 54 ofthe three boilers 10A, 10B and 10C of FIG. 5 are communicated with eachother via a cascade bus 55.

The cascading control system is operable, when the heat demand fallswithin the deadband, to turn the sequentially next boiler on after thesensed water temperature falls below the temperature range 142, and tomaintain the sequentially next boiler at a constant output, typicallyits minimum firing point, until the sensed water temperature exceeds anupper end of the temperature range 142, and to then turn off thesequentially next boiler.

Prevention of Flue Gas Condensation

A third issue addressed by the control system of the present inventionis the problem of condensation of water vapor from the combustionproducts exhausted from each of the boilers into the flue 46.

Non-modulating boilers are typically designed to operate constantly attheir maximum heat output which is typically designed so that theexhaust temperatures of the combustion products from the boiler will behigh enough to prevent condensation of water vapor from the exhaustgases. With modulating boilers, however, many prior art systemsencounter a problem in that when the heat output of a given boiler isturned down below a certain level the temperature of the exhaust gaseswill not be high enough to ensure that there is not condensation ofwater vapor as those exhaust gases pass through the vent flue. Thus manymodulating boilers typically require the use of expensive Category II orCategory IV venting materials. Furthermore, when Category IV ventingmaterials are required, each boiler is required by codes to be ventedseparately, instead of venting to a common vent. The reason the ventmaterials are expensive is that the flue temperature can get low enoughthat the water vapor in the flue product will condense within the flue.This condensation is acidic, and therefore the vent material mustwithstand this acidity. References to Category I, II or IV ventingmaterials are with reference to ANSI Z21.10.3 and ANSI Z21.13 standards.

To overcome this limitation, the present invention provides a fluetemperature sensor 44 (see FIG. 4) to measure the temperature of theflue gas products going into the flue. The control system of the presentinvention, then controls the modulation of the boiler to maintain theminimum flue temperature required to prevent condensation in the flue.For example, typically the exhaust gas temperature should be maintainedat a temperature of greater than about 300° F. to prevent condensation.

Thus for a typical boiler, the boiler may be required to be firing at alevel of for example 50%, which is substantially above its minimumfiring point of 25%, in order to maintain the exhaust gas temperaturehigh enough to prevent condensation. This will effectively reduce themodulating range of the boiler so that it must modulate between 50% and100% of its total output (i.e. a 2:1 turndown ratio) rather than beingable to modulate between 25% and 100% of its maximum output (i.e. a 4:1turndown ratio).

When utilizing the deadband control scheme of FIG. 6, this willeffectively increase the height of the deadbands 138 and 140 to extendfrom zero to 50% of the output of the modulating boiler rather than fromzero to 25% as was illustrated in FIG. 6.

This allows the boiler to compensate for the various conditions thatinfluence flue temperature, so that more modulation is available whenthese conditions are more favorable. Some of the conditions thatinfluence the flue gas temperature are the water temperature of thewater flowing through the heat exchanger of the boiler, the outdoortemperature of inlet air being drawn into the burner of the boiler, andthe flow rate of water going through the heat exchanger of the boiler.

By insuring that there will be no condensation of flue products, lessexpensive Category I flues can be utilized and all of the flues can bejoined together to a common exhaust which substantially reduces the costof the exhaust system from the boiler system.

Thus it is seen that the apparatus and methods of the present inventionreadily achieve the ends and advantages mentioned as well as thoseinherent therein. While certain preferred embodiments of the inventionhave been illustrated and described for purposes of the presentdisclosure, numerous changes in parts and steps may be made by thoseskilled in the art, which changes are encompassed within the scope andspirit of the present invention as defined by the appended claims.

1. A method of controlling a plurality of modulating water heaters forproviding hot water to a system at a water supply temperaturecorresponding to a temperature setpoint, the plurality of heatersincluding at least a first heater and a second heater, the first heaterhaving a modulatable output ranging from a first heater minimum outputgreater than zero to a first heater maximum output, the second heaterhaving a modulatable output ranging from a second heater minimum outputgreater than zero to a second heater maximum output, the plurality ofheaters having a combined range of output including a first modulatingrange extending from the minimum output of the first heater to themaximum output of the first heater, a deadband between maximum output ofthe first heater and the sum of the maximum output of the first heaterand the minimum output of the second heater, and a second modulatingrange extending from the deadband to the combined maximum output of thefirst and second heaters, the method comprising: (a) when a heat demandupon the plurality of heaters falls within the first modulating range,continuously modulating the first heater output to maintain the watersupply temperature at the temperature setpoint; (b) when the heat demandupon the plurality of heaters falls within the deadband, allowing thewater supply temperature to vary at least within a defined temperaturerange spanning the temperature setpoint before the second heater isturned on or off; and (c) when the heat demand upon the plurality ofheaters falls within the second modulating range, maintaining the firstheater at its maximum output and continuously modulating the secondheater output to maintain the water supply temperature at thetemperature setpoint.
 2. The method of claim 1, wherein during step (b)after the water supply temperature drops below the defined temperaturerange, the second heater is turned on and maintained at its minimumoutput until the water supply temperature exceeds an upper end of thedefined temperature range, and then the second heater is turned off. 3.The method of claim 1, wherein: during step (b), after the water supplytemperature drops below the defined temperature range the second heateris turned on after the heat demand on the plurality of heaters rises tothe sum of the maximum output of the first heater and the minimum outputof the second heater.
 4. The method of claim 1, wherein in step (b) thetemperature setpoint is the mid-point of the defined temperature range.5. The method of claim 4, wherein the defined temperature range is equalto the temperature setpoint plus or minus a constant in the range offrom 3° F. to 7° F.
 6. The method of claim 5, wherein the constant isabout 5° F.
 7. The method of claim 1, further comprising: during step(b), maintaining a firing rate of the first heater substantiallyconstant.
 8. The method of claim 1, wherein: the first heater minimumoutput is equal to the firing point of the first heater, and the secondheater minimum output is equal to the firing point of the second heater.9. The method of claim 1, further comprising: monitoring an exhaust gasoutlet temperature of the second heater; and wherein the minimum outputof the second heater is greater than a firing point of the second heaterand is sufficient to maintain the exhaust gas outlet temperature of thesecond heater high enough to prevent condensation of water vapor fromthe exhaust gas of the second heater.